A) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device having ferroelectric capacitors and its manufacture method.
B) Description of the Related Art
With the recent development of digital technologies, there arise high demands for processing or storing a large amount of data at high speed. Therefore, semiconductor devices used in electronic apparatus are required to have high integration density and high performance. For example, in order to realize high integration density of dynamic random access memories (DRAMs), techniques are widely researched and developed by which a ferroelectric material film or a high dielectric constant material film is used as a capacitor dielectric film, in place of a silicon oxide film or a silicon nitride film.
Vigorous researches and developments have been conducted on a ferroelectric memory (FeRAM) using a ferroelectric material film having spontaneous polarization characteristics as a capacitor dielectric film, in order to realize a nonvolatile memory capable of data read/write at lower voltage and at high speed.
A ferroelectric memory stores information by utilizing hysteresis characteristics of a ferroelectric capacitor having a ferroelectric film sandwiched between a pair of electrodes. The ferroelectric film generates polarization corresponding to a voltage applied between the electrodes, and retains spontaneous polarization even if the applied voltage is removed. As the polarity of the applied voltage is reversed, the polarity of spontaneous polarization is also reversed. Information can be read by detecting spontaneous polarization. As compared to a flash memory, a ferroelectric memory operates at a lower voltage and can write data at high speed with reduced power dissipation.
A capacitor dielectric film of FeRAM is made of lead zirconate titanate (PZT), La-doped PZT (PLZT), PZT-containing material doped with Ca, Sr or Si slightly, Bi-containing layer structure compound such as SrBi2Ta2O9 (SBT) and SrBi2(Ta, Nb)2O9 (SBTN) or the like, and manufactured by a sol-gel method, sputtering, metal organic chemical vapor deposition (MOCVD) or the like.
Generally, an oxide ferroelectric film in an amorphous or micro crystal state is formed on a lower electrode by the above-described film forming method, and thereafter heat treatment is performed to change the crystal structure to a perovskite structure or a bismuth compound layer structure. Therefore, the lower electrode of a ferroelectric capacitor is made of platinum group metal such as platinum (Pt) and iridium (Ir) hard to be oxidized in an oxygen atmosphere, or platinum group conductive oxide such as iridium oxide (IrOx). A ferroelectric film is likely to form oxygen vacancies by heat treatment in an oxygen atmosphere so that characteristics of the ferroelectric film are degraded, such as reduction in a switching charge and an increase in a leak current.
In manufacturing a ferroelectric capacitor, heat treatment in an oxygen atmosphere is required to be performed plural times in order to recover crystal defects such as oxygen vacancies. Therefore, the upper electrode of the ferroelectric capacitor is also made of platinum group metal such as Pt and Ir hard to be oxidized in an oxygen atmosphere, or conductive platinum group oxide such as IrOx and RuOx. Various proposals have been made using a laminated electrode of a plurality of electrode layers.
JP-A-2003-174146 proposes that an upper electrode is made of a first oxide electrode and a second oxide electrode, one being made of SrRuO which contains Pb, Bi, Cu at 0.1 at % or more and the other being made of IrOx.
JP-A-2004-247324 proposes that a PbRuO conductive layer is formed on a PZT film and a RuO2 conductive layer is formed on the PbRuO conductive layer to form an upper electrode.
Also in FeRAM, there are high demands for miniaturization and low voltage operation in recent years. It is therefore required that a ferroelectric film constituting a ferroelectric capacitor has a sufficient switching charge quantity QSW even in a miniaturized structure. If a multilayer wiring structure is adopted, there is a possibility that the characteristics of a ferroelectric capacitor are degraded by a reducing atmosphere process used by a multilayer wiring manufacture method.
If an upper electrode is made of a Pt film, an Ir film or the like, there is a non-negligible danger that hydrogen in a reducing atmosphere used by a process of forming an interlevel insulating film invades the Pt film or Ir film and is activated by a catalyst function of the metal film, and the activated hydrogen reduces the ferroelectric film. If the ferroelectric film is reduced, the operation characteristics of the ferroelectric capacitor are degraded considerably. This problem of degraded characteristics appears conspicuously particularly when the ferroelectric capacitor is made micro fine.
JP-A-2002-324894 (U.S. Pat. No. 3,661,850) proposes that a first upper electrode made of conductive oxide having a stoichiometric composition AOx1 at a composition of AOx2 is formed on a ferroelectric film, and a second upper electrode made of conductive oxide having a stoichiometric composition AOy1 at a composition of AOy2 is formed on the first upper electrode, where y2/y1>x2/x1. Namely, an oxidation degree of the first upper electrode is suppressed to form a good interface with ferroelectric material, and the second upper electrode is made of conductive oxide having an increased oxidation degree to suppress generation of metal functioning as catalyst. AOy2 has preferably a composition near the stoichiometric composition.